This invention relates to an improved Stirling engine and in particular to an improved free-piston Stirling engine having a hydraulic coupling to an output member such as a compressor of a heat pump.
The Stirling engine is a closed-cycle engine with cyclic recirculation of the working fluid. Power is produced by compressing the working fluid at a low temperature and expanding it at a high temperature. The required heat is added continually during expansion of the working gas inside the engine through a heat exchanger wall. Since this wall has a high heat capacity, it is not possible to rapidly heat and cool the same wall, therefore, the working gas is alternately shuttled between two stationary variable volume chambers in the working space, held respectively at high and low temperatures and called the hot space and the cold space.
The alternating heating and cooling of the same working gas would inherently waste quantities of heat, so a regenerator is placed between the hot and cold sources in the path of the working gas. Heat is stored in the regenerator as the gas moves from the hot space toward the cold space and is then released to the working gas as it passes back through the regenerator in moving form the cold space to the hot space.
The conventional Stirling engine includes two pistons: one, called the displacer, is a lightweight body mounted on a rod which moves the displacer to shuttle the working gas between the hot and cold spaces; the other, called the power piston, is of heavier construction and is responsible for the work transfer over the cycle.
The motions of the power and displacer pistons can be considered from a first order perspective, to give rise to three pressure wave components, two of which occur within the cold space or engine compression space. The first pressure component, called the power piston pressure wave, is associated with the motion of the power piston. Physically, this is the pressure wave which would exist in the engine if the displacer piston were held fixed and the power piston were oscillated sinusoidally. The amplitude of the power piston pressure wave is related to the springiness of the engine and is primarily a function of the engine pressure, enclosed volume, piston area, and piston mass.
The second component is associated with the motion of the displacer piston and is called the displacer piston pressure wave. Physically, this is the pressure wave which would exist in the compression space volume if the power piston were fixed and the displacer piston were oscillated sinusoidally. This wave is the result of two generally conflicting effects: the first is the change in pressure which results from moving the displacer rod in and out of the engine volume; the second is the change in pressure which occurs due to the change in gas temperature as the working fluid is shuttled between the hot and cold spaces in the engine. As the displacer moves toward the engine hot space, the first effect tends to increase the pressure and the second effect tends to decrease the pressure. For any practical engine operating point, the temperature effect more than offsets the volume effect. As a consequence, the displacer pressure wave leads the displacer motion by 180.degree..
The third component of the pressure wave occurs in the expansion space volume or the hot space and is due to seal leakage. This component results from the pressure drop across the seal and is proportional to the pressure amplitude, leading the pressure by 90.degree.. It is inimical to good engine efficiency and is the subject of considerable development effort to minimize. The sum of these three components is the pressure wave in the working space; if there were no pressure drop in the heat exchanger duct, this wave would lead the power piston motion.
The pressure wave components in the expansion and compression spaces may further be broken down into two elements: first, the basic pressure wave and, second, the pressure drop due to flow through the heat exchanger duct. The basic pressure wave approximates the pressure wave which would be measured in the middle of the heat exchanger duct. The expansion space pressure is the basic pressure plus the pressure drop between the middle of the heat exchanger duct and the expansion space. The compression space pressure is found in a similar manner. The forces which are exerted on the power piston and the displacer, because of the basic pressure wave, are obtained by multiplying the magnitude of this pressure wave by the area of the power piston face and the displacer rod area, respectively. These forces, which are 180.degree. out-of-phase with the pressure wave, are in a ratio of approximately 10:1. The power is proportional to the component of the force phasor which is normal to the displacement vector. As a consequence of the displacer rod area, the engine does feed power into the displacer through the rod area, and if the rod area is large enough, this power will exceed the power dissipated through the heat exchangers. The lag angle between the engine pressure wave and the power piston phasor is referred to as the engine pressure angle. A low pressure angle corresponds to a peaked or springy PV diagram while a high pressure angle corresponds to a more oval or flat PV diagram. From a thermodynamic perspective, a flat PV diagram is more desirable than a peaked PV diagram since the flat diagram has a lower peak-to-peak pressure ratio and hence, a smaller temperature variation of the gas in the compression and expansion space volume, and therefore, lower thermal mixing and thermal energy losses. The thermal mixing loss is the irreversibility which occurs when gas from the heater or cooler enters the expansion or compression space volume at a temperature significantly different from the gas temperature within the volume. The thermal entry loss is the loss which occurs when gas from the expansion or compression space enters the heater or cooler at a temperature significantly different from the heater or cooler metal temperature.
The unique feature of free-piston Stirling engines is that the piston motions are determined by the state of a balanced dynamic system of springs and masses, rather than a mechanical system.
The free-piston Stirling engine is an ideal vehicle to power residential-sized heat exchangers. It is extremely quiet, indeed virtually silent, in operation. It can be designed to be heated by any fuel whatsoever, and therefore is able to utilize the cheapest and most available fuel at any particular time. By using the same fuel for both heating and cooling, the seasonal demand on particular power sources can be substantially leveled to the benefit of the distribution system. The free-piston Stirling engine is sealed so the working fluids within the pressure vessel are not subject to loss through the shaft seals of conventional mechanical output Stirling engines. However, in a closed hermetic system utilizing more than one working fluid, it is necessary that they be separated. In addition, the lubrication within the sealed vessel must be maintained at the correct pressure and properly separated from the other working fluids, particularly the engine working fluid.
Power modulation of a Stirling engine heat pump alternator is theoretically controllable by controlling the pressure of the working gas in the engine. However, this also has the effect of altering the engine frequency which in turn can alter the frequency of the electric output of the system. In some situations, it may be desirable to regulate the power output while maintaining the system frequency constant.
As the power requirements for the heat pump increase in hot or cold weather, this condition must be sensed by the system which must automatically adjust the operating parameters to produce a higher output power. The conventional technique for accomplishing the power modulation is to adjust the time interval in which the compressor operates. This is inherently inefficient because of start-up power surges and the other known inefficiencies in operating a high-power output device intermittently to produce low power output levels. A much more efficient method would be to run a system continuously but modulate the input energy to produce a controlled output power as desired.